In cell phone services in recent years, enhancement of communication speed is getting more critical with a growing demand for data communications on top of voice communications. For example, in the GSM (Global System for Mobile communications) system in widespread use mainly in the European and Asian regions, voice communications have conventionally employed GMSK modulation that shifts the phase of a carrier in accordance with transmit data. Further, The EDGE (Enhanced Data rates for GSM Evolution) system has been proposed whereby data communications are also performed with 3π/8 rotating 8-PSK modulation (hereinafter referred to as 8-PSK modulation) including threefold bit information per symbol compared with GMSK modulation by shifting the phase and amplitude of a carrier in accordance with transmit data.
In a linear modulation system accompanied by amplitude fluctuation such as 8-PSK modulation, request for linearity of a PA (Power Amplifier) in a radio apparatus transmitter is rigorous. In general, power efficiency in the linear region of a power amplifier is lower than power efficiency in the saturation region. Thus, it has been difficult to provide high efficiency when related art quadrature modulation is applied to the linear modulation system.
(High Efficiency Attained by the Polar Modulation System)
Therefore a system for synthesizing amplitude modulations is known. This system includes steps of: dividing a transmit signal into a constant-amplitude phase signal and an amplitude signal; phase-modulating the transmit signal based on the constant-amplitude phase signal by using a phase modulator; and inputting a constant-amplitude phase modulation signal at the level a power amplifier performs saturation operation and driving a control voltage for the power amplifier at high speed. This system is referred to as the EER method (Envelope Elimination and Restoration) or polar modulation (polar modulating system or polar modulation system) and provides high efficiency of a power amplifier by way of the linear modulation system (for example, refer to Non-Patent Reference 1). In the description that follows, this system is called the polar modulation system in order to make it clear that the system is different from the quadrature modulation system.
FIG. 10 is a drawing where 8-PSK modulation signals used in the EDGE system are plotted on an IQ orthogonal coordinates. FIG. 11 is a drawing where amplitude component of the 8-PSK modulation signals in the 200 to 400 [μs] section extracted from one time-slot (577 [μs]) of GSM are plotted. In FIG. 11, the horizontal axis represents a time that has elapsed from the beginning of the time slot and the vertical axis the amplitude of an amplitude signal.
In order to represent an amplitude signal including a inflection points for the maximum value and minimum value of amplitude within 2 [μs] as shown in FIG. 11, a distortion compensation technique is indispensable to enhance the output response used for high-speed driving of a control voltage of a power amplifier.
(Distortion Compensation in the Polar Modulation System: Pre-Distortion System)
In a related art example of distortion compensation technique for enhancing the output response of a power amplifier in the polar modulation system, the control voltage characteristics of output signal amplitude and passing phase obtained beforehand in a saturation operation type power amplifier for a predetermined high frequency signal amplitude are stored into memory and distortion compensation of the pre-distortion system is performed while referencing the memory (for example, refer to Patent Reference 1).
FIG. 12 is a block diagram of related art polar modulation apparatus to which distortion compensation of the pre-distortion system described in Patent Reference 1 is applied. As shown in FIG. 12, the polar modulation apparatus comprises a power amplifier 1100, polar modulation unit 1101, a memory 1102, an amplitude controller 1105 including amplitude information correction unit 1103 and amplitude modulation unit 1104, and a phase modulation signal generator 1108 including phase information correction unit 1106 and phase modulation unit 1107.
The polar conversion unit 1101 separates an IQ signal input from a baseband signal generator section (not shown) into an amplitude signal r(t) and a phase signal θ(t) of a constant amplitude. Although not described in Patent Reference 1, normalization may be done so that r(t) have a maximum value of 1.
The memory 1102 stores the positive characteristics of an output signal amplitude characteristic (AM-AM: Amplitude Modulation to Amplitude Modulation conversion) and passing phase characteristic (AM-PM: Amplitude Modulation to Phase Modulation conversion) relative to an input control signal of the power amplifier 1100 for a predetermined input high frequency signal amplitude. The memory 1102 then outputs an amplitude correction signal and a phase correction signal having the inverse characteristic of the power amplifier 1100 in accordance with the input amplitude signal r(t). The inverse characteristic refers to a multiple of the inverse function of a positive characteristic by a predetermined constant.
The amplitude information correction unit 1103 corrects the input amplitude signal r(t) based on the amplitude correction signal output from the memory 1102. The amplitude modulation unit 1104 drives the control voltage for the power amplifier 1100 at high speed based on an output signal from the amplitude information correction unit 1103.
The phase information correction unit 1106 corrects the input phase signal based on the phase correction signal output from the memory 1102. The phase modulation unit 1107 performs phase modulation based on an output signal from the phase information correction unit 1106.
In this way, in consideration of the inverse characteristic of the output characteristic relative to the input control signal of a power amplifier, the pre-distorted amplitude modulation signal and phase modulation signal have desired output amplitude and phase influenced by actual amplitude compression (AM-AM distortion) and cross modulation (AM-PM distortion) generated by the power amplifier. This enhances the output response (linearity) to the input control voltage.
(AM-AM Characteristic in a Polar Modulation System: not Passing Through the Origin)
Although not disclosed in Patent Reference 1, an example of characteristic stored in the memory 1102 will be described. The data for AM-AM characteristic compensation stored in the memory 1102 is created based on the output signal amplitude characteristic (AM-AM) for the input control signal under the conditions that the input high frequency signal amplitude is constant. The data is obtained at the time the output signal amplitude is stabilized after the control signal is input. The AM-AM characteristic is represented by a curve that does not pass through the origin even when the linear region is extended, that is, a curve shown by the solid line in FIG. 13 in some device structure of a power amplifier.
In FIG. 13, the horizontal axis represents a control voltage applied to the power amplifier 1100 and the vertical axis an output voltage from the power amplifier 1100. The curve (A) shown by a solid line in the figure represents the output voltage characteristic of a fundamental frequency obtained in case a predetermined control voltage is applied. The straight line (A) shown by a dotted line represents the characteristic of an extended linear region of the curve.
Thus, the data stored in the memory 1102 for AM-AM characteristic compensation corresponds to points sampled at predetermined intervals of the curve (A) shown by the solid line in FIG. 13 (intersection of straight lines normal to the vertical axis and the curve).
Next, an example of distortion compensation processing in a polar modulation apparatus to which distortion compensation of pre-distortion system (AM-AM characteristic) is applied is shown in FIG. 14. With taking the amplitude signal shown in FIG. 11 as an example, FIG. 14 shows the relationship between a control voltage and an amplitude signal used to find a control voltage to be applied to the power amplifier 1100 when an amplitude signal is represented by the inverse characteristic of the output signal amplitude characteristic (AM-AM) for the input control signal shown in FIG. 13, that is, by the output of the power amplifier 1100.
In FIG. 14, the horizontal axis represents the control voltage applied to the power amplifier 1100 and the vertical axis the output voltage from the power amplifier 1100. The curve (A) shown by a solid line in the figure represents the output voltage characteristic of a fundamental frequency obtained in case a predetermined control voltage is applied. The straight line (A) shown by a dotted line represents a characteristic of an extended linear region of the curve.
In case the sampling interval of data stored in memory is narrow relative to the output signal amplitude characteristic (AM-AM) for the input control signal in FIG. 13 or 14, it is possible to accurately represent the amplitude signal, that is, ensure compensation accuracy with a tradeoff that the capacity of data stored in memory increases. In case the sampling interval of data stored in memory is broad, it is possible to reduce the capacity of data stored in memory with a tradeoff that the compensation accuracy is degraded.
Thus, in distortion compensation of the pre-distortion system, it is important to reduce the capacity of data stored in memory while assuring the compensation accuracy. It is also important to suppress an increase in the circuit scale related to distortion compensation.
(Distortion Compensation in a Quadrature Modulation System 1: Setting a Broad Sampling Interval of a Linear Region)
In a first exemplary related art method for reducing the capacity of data stored in memory while assuring the compensation accuracy with respect to distortion compensation of the pre-distortion system (AM-AM characteristic) in the quadrature modulation system, a broad sampling interval of compensation data is set to a region where the characteristic of the power amplifier is linear while a narrow sampling interval of compensation data is set to a region where the characteristic of the power amplifier is nonlinear. In this approach, the input signal is converted to addresses used for access to memory (for example, refer to Patent Reference 2).
FIG. 15 is a block diagram showing a related art distortion compensation circuit of the pre-distortion system in the quadrature modulation system described in Patent Reference 2. As shown in FIG. 15, a distortion compensation circuit 1400 comprises an amplitude component extraction circuit 1401, a memory 1402, correction unit 1403 and an address calculation circuit 1404.
(AM-AM Characteristic in a Quadrature Coordinate Modulation System: Passing Through the Origin)
The amplitude extraction circuit 1401 extracts the amplitude component (amplitude value (a)) of a modulating signal from the IQ signal input by a baseband signal generator (not shown). The memory 1402 stores the positive characteristics of the output amplitude characteristic (AM-AM) for the input high frequency signal and passing phase characteristic (AM-PM) for the input high frequency signal of a power amplifier shown by the solid line and dotted line in FIG. 16 under the conditions that the control voltage is constant. The memory 1402 then outputs a correction signal of the inverse characteristic of the power amplifier in accordance with an address value (b) output from the address calculation circuit 1404.
In FIG. 16, the horizontal axis represents the amplitude of an input high frequency signal, the vertical axis (left) the amplitude of an output signal, and the vertical axis (right) the phase rotation volume (passing phase) of the output signal with respect to the input high frequency signal as a reference. In general, the change in the passing phase is small in the linear region of the output signal amplitude characteristic relative to the input high frequency signal and is large in the nonlinear region.
The correction unit 1403 corrects an IQ signal input from a baseband signal generator (not shown) based on the correction signal. The address calculation circuit 1404 obtains an address value (b) as a reference value used in an access to data stored in the memory 1402, from the amplitude value (a) output from the amplitude component extraction circuit 1401.
In distortion compensation of the pre-distortion system in a quadrature modulation system, a multiplication circuit for performing complex multiplication as correction unit 1403 is typically used. The multiplication circuit multiplies the IQ signal input from the baseband signal generator (not shown) by a correction signal as an inverse characteristic of a power amplifier.
In case a characteristic curve passes through the origin such as the case of the output signal amplitude characteristic (AM-AM) for the input high frequency signal as shown by a solid line in FIG. 16, a multiplier coefficient by which an amplitude component is multiplied is constant and a change in the passing phase (AM-PM) is small in a linear region where the gradient is constant. Thus, it is possible to broaden the sampling interval of correction data stored in the memory 1402 compared with the sampling interval in a nonlinear region without degrading the compensation accuracy.
The memory 1402 uses the above relationship to broaden the sampling interval in a linear region and narrows the sampling interval in a nonlinear region to store correction data. The address calculation circuit 1404 performs conversion of sampling intervals in order to compensate for inconstancy of sampling interval of data stored in the memory 1402.
The above conversion processing will be described referring to FIGS. 17 and 18 described in Patent Reference 2. FIG. 17 shows the conversion processing of sampling interval in the address calculation circuit 1404 described in Patent Reference 2.
In FIG. 17, the left column shows amplitude values (a) output from the amplitude component extraction circuit 1401 and sampled at constant intervals. The right column shows address values (b) output to the memory 1402, with plural amplitude values (a) corresponding to a single address value (b).
FIG. 18 shows an example of correction signal stored in the memory 1402. In FIG. 18, the left column shows address values (b) output from the address calculation circuit 1404 and the right column shows correction signals to be output to the correction unit 1403.
As shown in FIG. 17, by retaining as a table the correspondence between amplitude values (a) and address values (b), formulating the correspondence or making judgment of conditions for the correspondence, it is possible to convert sampling intervals. As shown in FIG. 18, the compensation data volume stored in the memory 1402 is smaller than that obtained when sampling is made at constant intervals in both the linear and nonlinear regions.
(Distortion Compensation in a Quadrature Modulation System 2: Storing a Difference from an Approximation Function into Memory)
In a second exemplary related art method for reducing the capacity of data stored in memory while assuring the compensation accuracy with respect to distortion compensation of the pre-distortion system (AM-AM characteristic) in a quadrature modulation system, any portion of the output signal amplitude characteristic (AM-AM) relative to the input high frequency signal of a power amplifier that may be subjected to linear function approximation is approximated as such, and a difference between the approximation function and the original characteristic is stored into memory (for example, refer to Patent Reference 3).
FIG. 19 is a block diagram showing a related art distortion compensation circuit of the pre-distortion system in the quadrature modulation system described in Patent Reference 3. As shown in FIG. 19, the distortion compensation circuit comprises a polar conversion unit 1101, an adder circuit 1103 as amplitude information correction unit, a phase information correction unit 1106, an approximation function adder section 1801, and a memory 1802. Although not shown, an output amplitude signal from the adder circuit 1103 is synthesized with an output phase signal from the phase correction unit 1106 and the resulting composite signal is converted to an IQ signal format again. The re-converted IQ signal is input to a quadrature modulator (not shown). Thus, similarly to the data stored in the memory 1402 shown in FIG. 15, the data stored in the memory 1802 is the positive characteristics of the output amplitude characteristic (AM-AM) and passing phase characteristic (AM-PM) relative to the input high frequency signal of a power amplifier under the conditions that the control voltage is constant.
The relationship between the input signal r(t) and the output signal r2(t) in the approximation function adder section 1801 is represented by the expression:r2(t)=A×r(t)+B. 
For the same portion as that in FIG. 12 that explains related art polar modulation apparatus to which distortion compensation of the pre-distortion system is applied that is described in Patent Reference 1, the same signs are given and corresponding description is omitted.
FIG. 20 shows the roles of the approximation function adder section 1801 and the memory 1802 described in Patent Reference 3. In FIG. 20, the horizontal axis represents the amplitude of an input high frequency signal and the vertical axis the amplitude of an output signal. The solid line in FIG. 20 represents the inverse characteristic of the output signal amplitude characteristic (AM-AM) relative to the input high frequency signal of a power amplifier. The dotted line shows an approximation function represented by a linear function (y=Ax+B, B=0) and added in the approximation function adder section 1801. The memory 1802 stores a difference between the inverse characteristic of AM-AM and the approximation function.
In this way, by using both the linear function approximation in the approximation function adder section 1801 and storage of a difference data between the desired characteristic and the approximation function into the memory 1802, it is possible to reduce compensation data in a linear region that can be represented by an approximation function stored in the memory 1802. This reduces the capacity of data stored in the memory 1802 compared with a case where the correction data in all regions including the linear region into the memory 1802.
Patent Reference 1: JP-T-2004-501527 (FIG. 10)
Patent Reference 2: JP-A-2002-135349 (FIGS. 1, 2)
Patent Reference 3: JP-A-2000-201099 (FIGS. 1, 3a, 3b)
Non-Patent Reference 1: Kenington, Peter B, “High-Linearity RF Amplifier Design”, Artech House Publishers (FIG. 7.1, Page 427)